There is an increasing interest in nano-sized materials in numerous technical applications. Such nano-structured materials are cornerstones in many attempts to develop and exploit nanotechnology. They exhibit properties, which are significantly different from those of the same materials of larger size. During the last decade, the insight in to nano-structured materials has dramatically improved through the application of new experimental methods for characterization of materials at the nano-scale. This has resulted in the synthesis of unique new materials with unprecedented functional properties. For nano-structured coatings, physical properties such as elastic modulus, strength, hardness, ductility, diffusivity, and thermal expansion coefficient can be manipulated based on nano-meter control of the primary particle or grain size. For nano structured powders parameters such as the surface area, solubility, electronic structure and thermal conductivity are uniquely size dependent.
The novel properties of such nano-structured materials can be exploited and numerous new applications can be developed by using them in different industries. Examples of potential applications include new materials such as improved thermoelectric materials, electronics, coatings, semiconductors, high temperature superconductors, optical fibres, optical barriers, photographic materials, organic crystals, magnetic materials, shape changing alloys, polymers, conducting polymers, ceramics, catalysts, electronics, paints, coatings, lubricants, pesticides, thin films, composite materials, foods, food additives, antimicrobials, sunscreens, solar cells, cosmetics, drug delivery systems for controlled release and targeting, etc.
Addressing and exploiting such promising applications with new materials generally requires an improved price-performance ratio for the production of such nanostructured materials. The key parameters determining the performance are the primary particle (grain) size, size distribution of the primary particles, chemical composition and chemical purity as well as the surface area of powders, while the primary parameters for in relation to price are the ease of processing and suitability for mass production.
Several techniques have been used in the past for the manufacture of micron- or nano sized particles. Conventional techniques for submicron powders include spray drying, freeze drying, milling and fluid grinding, which are capable of producing powders in the micrometer range. Manufacturing techniques for producing submicron materials include high temperature vapour phase techniques such as flame synthesis and plasma arc methods, which allow production of nano-scaled powders consisting of hard or soft agglomerates of primary particles.
Solution sol-gel and hydrothermal synthesis are the major low temperature processes for production of fine particles with nano-scaled primary particles or grains.
Sol-gel processing is widely used as it is a versatile technology that allows production of homogeneous high purity fine particles with a relatively small primary particle size to be produced from numerous materials in the form of powders, films, fibres, spheres, monoliths, aerogels, xerogels as well as coatings. The precursors can be metal organics, metals, inorganic salts etc.
The key drawbacks from the sol-gel process are that it is time consuming, and need after treatment such as drying and calcinations. In the traditional sol-gel process, it is necessary to calcine the product for up to 24 hours in order to obtain a crystalline product. In addition to a higher energy usage and a more complicated process this has the unfortunate effect that substantially growth of primary particles occurs, and that the specific surface area may be decreased by up to 80%.
Hydrothermal processing have been used for batch synthesis of a wide range fine such oxide powders such as nano-sized materials in nearly a century. The term hydrothermal relates to the use of water as reaction medium and regime of high pressure and the medium to high temperature applied. A major drawback is the relatively long reaction and aging time e.g. hours to days required at low to medium temperatures and the very corrosive environment at higher temperature. Further, the characteristics of said nano-sized products is greatly influenced by factors such as heating rate, temperature, concentrations of precursors and/or reactants. Typical the result is a product having a wide particle size distribution, and it is difficult to obtain a uniform product with well defined characteristics in the nanoregime.
Arai and Adschiri (U.S. Pat. No. 5,635,154) discloses a process for production of fine oxide particles by thermal decomposition of metal salts in water at sub- or supercritical conditions. The process comprises pumping a premixed fluid containing a metal salt into a pipe having a heating and a subsequent cooling zone. At the end of the pipe a discharge valve is arranged through which the produced material is discharged to a collecting chamber. The process may be performed in a continuous or semi-continuous mode and may result in nano sized materials for certain compounds and reactions conditions. However, though it do not disclose information of important characteristics such as primary particle size and secondary particle size, particle size distribution/-s, and how such characteristics is controlled. No information related to mixing is given and the process has several drawbacks.
Pessey et al (U.S. Pat. No. 0,203,207A1) discloses a coating process at near critical or supercritical conditions. Particles to be coated are kept dispersed in a reaction mixture with at least one precursor is dissolved in at least one solvent and brought under supercritical or slightly sub-critical pressure and temperature, and subsequently a conversion of said precursor or precursors is caused by increasing the temperature above the thermal decomposition temperature and/or by the action of a suitable reactant, whereby a film of a coating material is deposited on the surface of said particles, whereafter the fluid is brought into a gaseous state in order to remove the solvent.
Lester et al (WO 2005/077505A2) discloses a counter current mixing reactor for use in continuous synthesis of nanoparticles of metals or metal oxides in high temperature water with improved particle size and shape compared to previous designs of reactors. The mixing is disclosed as being between a heated pressurized or supercritical fluid and a denser fluid and the disclosure relates to a specific design of a mixing chamber.
Although the known methods and devices may have the potential to produce nano sized material, they still suffer from being able to efficiently producing a uniformly sized nano material and the devices used for producing the materials are typically blocked by the material being formed. In connection with the present invention it has been found that a commonly occurring cause to non-uniform size distribution and blocking stems from application of heat and/or cooling to obtain the required process conditions for formation of nano materials. The heating and/or cooling may be performed during or after mixing of fluids when a continuous production is performed or when a bath-process is performed to bring the fluid up to the required process conditions.
Typically, the fluid in which the reactions resulting in formation of nano materials is to occur is heated by adding heat to e.g. the wall of a reactor vessel in a heat-exchanger-like manner. Thereby a thermal boundary layer is generated inside the reactor where for instance                the temperature of the fluid close to the wall of the reactor is so high that the reactants are destroyed,        the temperature of the fluid in the center of the reactor is so low that unwanted reactions takes place.        
In addition to this, the time needed for heating the fluid is typically so long that, again, unwanted reactions take place.
Furthermore, the combination of applying heat to a mixing device, where two fluids are mixed to form nano materials, in a heat exchanger like manner results also in a thermal boundary layer. The effect is typically that nano materials are produced in the fluid mixture only in the vicinity of the source of heat (typically the walls of the mixing device). Such locally formation of nano materials close to a surface results often in depositing of nano materials on the surface—in particular as the nano material formed precipitates from the fluid—resulting in blockage of the mixing device.
Although great effect may be put into the design of heat exchangers to minimize the thermal boundary layer precipitation and deposition on surfaces may still occur which tends to block the flow passages.
Thus, while many of the processes and devices suggested earlier have shown to be able to produce nano sized materials in short production runs and at laboratory scale, they still seem to suffer from not being scalable to longer production runs and with a higher output.